The composition of natural gas and other hydrocarbon fuel gases is often identified to determine its energy content, heating value, specific gravity, Wobbe index, and/or other properties. The composition measurement may be performed during exploration and mining, manufacturing processes in refineries, as part of quality assurance/control during buy/sell transactions, as well as during its use in power or electricity generation. Composition is typically determined using gas chromatography (GC), where physical separation of the constituents of the sample gas is performed by flowing the gas sample through a thin long column. After the constituents of the sample gas move through the GC column, they reach a detector that provides a measured response proportional to the concentration of each compound. Drawbacks of current GC techniques include: (i) long separation time and thus long measurement time, (ii) requirement of a carrier gas which may be expensive to obtain and maintain, and (iii) poor measurement stability, requiring frequent recalibration.
Moreover, hydrocarbon gases absorb electromagnetic light radiation in several wavelength bands in the near infrared and in the mid infrared. The absorption signals may be used to perform correlation analysis with the energy content or heating value of the sample hydrocarbon gas, as described, for example, in U.S. Pat. No. 6,555,820 to Tacke et. al and U.S. Pat. No. 5,822,058 to Adler-Golden et. al. Many products in the market, to-date, use infrared absorption signals of hydrocarbon gases to determine presence of combustible gases and vapors in the ambient or surrounding air. However, spectral speciation of the constituents of a hydrocarbon gas mixture is a challenging task given the overlapping nature of the hydrocarbon spectra. FIG. 2 shows example absorption spectra of C1-C4 alkane gases in the near infrared (NIR), specifically between 1600 nm and 1900 nm. As shown, although the individual constituents have unique shapes or spectral features, they significantly overlap one another. Furthermore, the spectra of C2, C3 and C4's (ethane, propane and butanes) are broad and have limited distinguishing features from each other.
Thus, there is a need for an improved method and system for monitoring the composition of hydrocarbon gases.